A point in space $(x,y,z)$ is randomly selected so that $-1\le x \le 1$,$-1\le y \le 1$,$-1\le z \le 1$.  What is the probability that $x^2+y^2+z^2\le 1$?
The region that the point $(x,y,z)$ can lie in is a cube with side length 2.  It has total volume of $2^3=8$.  The region of points that satisfy $x^2+y^2+z^2\le 1$ corresponds to a unit sphere centered at the origin.  The volume of this sphere is $\frac{4\pi}{3}\cdot 1^3=\frac{4\pi}{3}$.  This sphere lies completely inside, and is tangent to, the cube.  The probability that a point randomly selected from the cube will lie inside this sphere is equal to $\frac{\frac{4\pi}{3}}{8}=\boxed{\frac{\pi}{6}}$.